Speaking in Edinburgh, where he was campaigning for next month's Scottish Parliament elections, Mr Hague said: "We think that it may be justified, but we want to see clear and achievable objectives.

"Are we absolutely clear, is the government clear, is Nato clear what the objectives of this ground force should be?"

Mr Hague said that Nato was now in a quite different situation than when the air strikes began.

"We supported air strikes on the basis of the government's assurances. Clearly this has gone on much longer than was initially expected."

The Tory leader said the humanitarian crisis was also far greater than anyone had anticipated. He added: "Whatever happens this must succeed.

"That is why the prime minister announced that we would have to look at all the other options."

William Hague called for clear objectives for the use of ground troops

Earlier another senior Conservative, former chancellor Kenneth Clarke, said of the Kosovo conflict: "I do think the answer will now require ground troops."

He told BBC News Online the government should decide "precisely what its position is on ground troops and not keep sounding off day by day, causing confusion about what it wishes to do".

Mr Clarke said ministers should focus instead on planning with Nato allies how troops would be deployed. "You can announce you're using ground troops on the day they enter Kosovo," he said.

Salmond restates opposition

The Tory backing for ground troops followed Alex Salmond's restatement of his belief that Nato's bombing campaign against Serbia is wrong.

Alex Salmond: Backs the armed forces but not the conflict

The Scottish National Party leader said the Nato campaign had made a terrible situation "a hundred times worse".

Speaking on BBC One's Question Time programme, Mr Salmond said Nato's action against Yugoslav President Slobodan Milosevic was "of dubious legality but above all one of unpardonable folly".

Mr Salmond said President Milosevic was a "gangster", but he did not doubt Nato's action "wasn't done for the highest of motives".

He added: "Just because something must be done is not a reason for doing the wrong thing and I think the bombing campaign was the wrong thing."